Overhead projectors are in common use in schools, and businesses, and at conferences, and in fact, are used more frequently than slide projectors nowadays. In using an overhead projector the transparency sheets are generally stored in a pile beside the projector, and each sheet is removed from the pile, as needed, and placed on the projector platen; when finished with each sheet, it is removed from the platen and placed in a different pile, and this sequence is then repeated with each successive transparency. This series of steps can sometimes become quite awkward, and transparencies can get mixed up, particularly when the user is preoccupied with the presentation, or not totally at ease during a formal presentation.
A common problem observed in using overhead projectors is that a transparency is inadvertently projected onto a screen in a rotated or inverted configuration, and tiffs is often followed by several attempts at remedying the problem before it is corrected. This can happen, under the pressure of a presentation, even when all transparencies are initially stacked in an orderly arrangement. Frequently, at the end of a presentation, the transparencies are scattered all over a table. On the other hand, it is common to find that the speaker is not provided with sufficient space to properly hold both the initial stack of transparencies and the stack of `used` transparencies. For example, many overhead projector carts have room only for the projector with no additional space for conveniently holding transparencies during a presentation. As an indication of the discomfort which some individuals feel when using an overhead projector, they sometimes have another person actually `operate` the projector for them by transferring the transparencies to and from the projector platen as needed.
The prior art contains several devices for dispensing transparencies to overhead projectors. For example, the following patents describe automatic, motor-driven dispensers of transparencies for overhead projectors: U.S. Pat. No. 5,172,145 to Stephenson (1992); U.S. Pat. No. 4,944,586 to Rightmyre (1990); U.S. Pat. No. 4,942,411 to Polston (1990); U.S. Pat. No. 4,756,616 to Min et al. (1988); U.S. Pat. No. 4,707,092 to Mindell (1987); U.S. Pat. No. 4,264,158 to Lindqvist (1981); U.S. Pat. No. 3,778,143 to Treher (1973); and U.S. Pat. No. 3,594,082 to Lonchar (1971). All of those devices are relatively complicated, and are subject to mechanical problems because of the mechanisms involved. The complicated mechanisms in those prior-art devices also make them more expensive. In addition, those devices .require a power supply and are generally quite bulky. Some of these prior-art accessories are considerably more complex than the overhead projectors themselves. The following patents describe dispensers of transparencies for overhead projectors that are manually operated: U.S. Pat. No. 5,198,846 to Zilber (1993); U.S. Pat. No. 4,715,705 to Nord (1987); U.S. Pat. No. 4,203,659 to Constantine et al. (1980); U.S. Pat. No. 3,609,026 to Verebay (1971); U.S. Pat. No. 3,600,079 to Smith etal.(1971); U.S. Pat. No. 3,536,393 to Kitch (1970); U.S. Pat. No. 3,524,703 to Wright (1970); and U.S. Pat. No. 3,438,703 to Winnemann (1969). These prior-art manually-operated devices have several disadvantages. For example, the devices described in U.S. Pat. Nos. 5,198,846 and 4,715,705 are quite bulky and cumbersome; and the devices described in U.S. Pat. Nos. 4,203,659, 3,609,026, 3,600,079, 3,536,393, 3,524,703 and 3,438,703 require that the transparencies be constrained by means of a ring binder. Some of the prior-art devices require that the transparencies have holes punched in them or that they have special flanges, or that the transparencies be mounted in special holders. In addition, many of the manually-operated devices described in these prior-art patents require relatively awkward manipulations during operation of the overhead projector.
Zilber's patent (U.S. Pat. No. 5,198,846) describes a manual cassette system for overhead projection transparencies that includes a storage and dispensing cassette for holding an initial stack of transparencies on one side of an overhead projector and a receptacle on the opposite side of the projector for receiving transparencies following their projection. This device is quite bulky and requires a mounting frame and a container for holding transparencies on two sides of the projector. In addition, the storage and dispensing cassette of U.S. Pat. No. 5,198,846 is relatively complex involving a movable follower plate for supporting transparencies, a biassing means for pushing the follower plate with superposed transparencies upward against the top of the cassette, a movable door interconnected to the follower plate, and a narrow discharge slot through which the transparencies are passed. Thus, this is a multicomponent device requiring the manufacture of several individual components and their assembly into the final product. This makes the device more expensive to manufacture and also affords additional opportunity for malfunction. In addition, the initial stack of transparencies must be inserted through a door at one end of the dispensing cassette which leads to an awkwardness and delay in setting up the device for use. A batch of transparencies cannot be directly removed from the storage cassette, but the door must first be opened and the batch of transparencies removed through that door. Furthermore, individual transparencies may be scratched during transfer to the projector due to contact under pressure with the top of the cassette, and during transfer of transparencies through the discharge slot of the cassette.